Modulation system



June 10, 1941. A ALFQRD 2,244,756

MODULATION SYSTEM Filed Aug. 5, 1939 2 Sheets-Sheet 2 F IGA.

- Y lNvr-:NTOR ANDREW AL ,P0/9p ATTO RNEY Patented June io, 1941 MODULATION SYSTEM Andrew Alford, New York, N.

Y., assignor .tom-

ternational Telephone Development Co., Inc.,V

New York, N. Y., a corporation of Delaware My invention relates to radio beacons and more particularly to arrangements for coupling to radio beacon antennae to prevent intermodulation of signals.

In practice radio beacons are often arranged so that the antennae are energized over branch transmission lines from a signal carrier source.l

'I'he different portions of the signal energy transmitted from each of the directive antennae are given a characteristic modulation by some means usually in the transmission line which alters the amount of energy transmitted to the antennae so as to produce a modulation. This modulation may be obtained by use of rotatable condensers or other forms of impedance changing devices in the lines feeding the antennae.

In these known types of systems, a commo'n dimculty is that which may be termed crossmodulation. Thus when the impedance of one line is changed, not only does the current in the particular antenna to which it is connected become modulated, but the resultant load impedance is also varied lation is transferred to the other antenna. Similarly, the modulation in the second antenna is reflected into the rst antenna so that there is quite an amount of cross-modulation. Since the modulation frequencies commonly used are in the order of 90 cycles per second and l0() cycles per second, the impedance cannot oe coordinated with ,the minimums of desired wav and the cross-modulation cannot be completely prevented. This-results in a course beam which is less definite than would otherwise be obtained and is more subject to extraneous eects.

In accordance with my invention. a. balancing network is interposed between the source of energy and the transmission lines connected to the antennae to which the modulators are coupled, so that cross-modulation is prevented.

According to another feature of my invention, the network used for preventing cross-modulation is designed so as' to Working condition so that a very small percentage of energy is wasted.

Other features and objects of my invention will be clear from .the description of a preferred so that some oi the modupoint on lines Mii, iii, are connected provide for an optimum embodiment of my invention made in connection with the accompanying drawings, in which- Fig. 1 d'iagrammatically illustrates an arrangement in accordance with my invention;

Fig. 2 diagrammatically illustrates' a particular preferred embodiment of my invention, and Figs. S'and d illustrate modulation arrangements that may be utilized to `attain these desired results in accordance with the systems of my invention.

In Fig. l, an energy source it@ is shcwneonnected over a transmission line lili :to a network indicated generally-at H02. Network it?? is a special form of the high frequencybridge such. as illustrated in .my prior United States Patent No. 2,147,809 of February 21, 1939. This network is made in the form o' reentrant loops having e.. phase changing arrangement such. as .the trans- DOSition shown at |03. The network is formed with four arms of substantially equal. electrical length so that the whole arrangement has the appearance of a bridge. At the intersection ci.' two of the arms, such as |04, ddii, is connected the transmission line lili and across the diagonally related junction of arms itil, tilt is connected a balancing network tilt. This balancing impedance network has preferably an 1inpedance substantially equal to the surge impedance of the load looking into it at the point of connection to the bridge. Across the other diagonally related junction point of the bridge network are connected transmission lines iii, to the terminals of which. are connected high frequency loads M2, till, which may constitute directive antennae. At an :Intermediate odulael .tted

tors tit, ii ii, for giving to the energy over lines itil, iii, a characteristic frequency.

When energy from source im! is .impressed upon. the bridge, it is clear that so long as the im pedance of loads M2, it@ together with ne transmission lines H0, iii are eguall gy istransmitted into the balancing impede ,e i dit, because of the symmetrical relation.. .Io-Wever, when modulators -i I4, H5 do not have the same effect on each line, the loads are not equal and therefore some of the energy from source itil is carried to the balancing network impedance itt. However, this change in impedance oi load, for example, load HZ, cannot be carried through to load H3 since the impedance of the source |00 and the balancing impedance it@ are equal and the network is therefore balanced with respect to this energy. It is therefore clear that with an arrangement of this type changes in the apparent load impedance due to the, .inodulation cannot be reected from one antenna to :the other, and as a result cross-modulation is avoided.

Although the arrangement described above :ls primarily useful for systems wherein energy .from a single source, characterized by diierent sig- 'nals is to be transmitted to separate loads, the arrangement may also be used for transmitting from two separate sources placed in relative positions such as shown in loads IIZ. H3, connected to a single load arrangement at a point such as I I without causing cross-modulation between the sources. This is clear from the general reciprocity law Jrelating to such circuits.

Under the conditions of the worst unbalance, that is, when for example, network |I4 is tuned so as to transmit no energy to antenna ||2 and the energy transmitted to 1| 3 is maximum, the following results are noted. Since network I I4 produces an effective open circuit in line ||0,

junction point b is bridged by an open quarter Furthermore, where a simple balancing netl work is desired any desired form of phase shifting networks .may be utilized in place of the transposition shown at |03. For example, an additional half wavelength of transmission line may be added, or any other well known form of.

phase shifter may be provided. lThese various forms of phase Shifters are disclosed in my previously mentioned Patent 2,147,809.

In Fig. `2 is illustrated a preferred embodiment of my invention applied to a radio beacon circuit. In this arrangement the energy source |00 terminates in a tuned output circuit 200, which is dcoupled over a tuned secondary circuit to transmission line |0I. Arms |06, |05, |06, H01 of network |02are each made equal to a quarter wavelength of the operating frequency or an odd multiple thereof in length. The network I 08 has an impedance Zo equal to the surge impedance of .the load looking into at point a.. The four junction points of the bridge network are designated a, b, c, d, as shown. The modulating networks IM, IIE comprise sections of transmission line coupled to the antenna leads I I0, III and provided with rotatable condenser plates 203, cut so as to give a diiferent modulation to the energy transmitted from each or antennae H2, H3. When these networks are tuned by their respective condensers so as to he in resonance with the carrier frequency they act as cut-olf filters producing an apparent open circuit inthe line. A common driving motor 204 connected by means of a shaft, as shown by broken lines to the condenser rotors, serves to drive these condensers so as to modulate energy at two different frequencies, for example, at frequencies 90 and 100 cycles, respectively.

The impedance of loads I I2, |I3 as seen from points b and d, respectively, may be so related as to be equal to one-half Zn. Then, since arms |04, |05 are quarter wavelength lines, the impedance at point a will be twice the surge impedance of line |0| or 2 Zo. Thus, since Zu is working in two branches in parallel, am impedance match between the source and the load is provided. If

then, the modulating arrangements are located at such a distance from the junction points c and d, that the portion of the lines 0, III between points c and d and the modulator point, where the apparent open'circuit occurs when the networks are tuned to stop the ilow of current in lines IIO, III is made equal to an odd multiple of a quarter wavelength. The following operational characteristics will be observed:

At the time loads I i2, Il I3 are drawing the same amount of power there will be a node at point c,

so that the arms |06, iii? will be equivalent to a pair of short circuited quarter wavelength lines hung on the load, and no e'ect upon the impedance of the circuit will be caused thereby. Therefore, at-this time no current will flow into network |08 and no energy will 'be consumed therein. At the time that neither antenna I I2 or ||3 are drawing any power, that is, when both are effectively open circuited, by modulators H,

||5, the network |02 is completely balanced and no energy will be dissipated in network |08.

` duce a relatively small wave section 'of transmission line. quarter wave line has the same effect to the energy as if a short circuit across the network at point b. Therefore, arm |04 acts' as a short circuited quarter wave transmission line across point a, and offers substantially infinite impedance to energy, so that all of the energy from source I 0| flows into arm |05. At the same time arm |01 acts as a short circuited quarter wavelength line across point c and does not eiect the impedance network. Since antenna H3 and transmission line has an effective impedance equal to one-half Zu and a network |08 has an impedance of Zu, two-thirds of the energy from source |0I will be transmitted to antenna H3 and only one-third oi? the energy will be dissipated in impedance |08.

Since the times at which this worst condition will arise are not very'frequent, an arrangement such as shown in Fig. 2' with the optimum dimension will dissipate only from 6% to 12% of the energy. In any case where such precautions are not taken, the worst condition will result in half of the energy being dissipated in network |08, at the time antenna ||2 or antenna 3 is carrying all of the load. Even this arrangement will prodissipation of the total power from the transmitter. Thus, although the arrangement according to my invention results in a certain amount of waste power, this is not a very important factor when it is considered that if modlating tubes are used inefficient couplings must b'e provided in order to reduce crossmodu. lation, resulting also in ya waste of power. Furthermore, in these other types of arrangements a complete balancing of the circuit and elimination of cross-modulation cannot be achieved, as it can in accordance'with my invention.

By utilizing the arrangement with the optimum operating characteristics as described in connection with Fig. 2, and also providing that the transmission line i0| is made equal to a quarter of a wavelength or odd multiple thereof, further advantages may be achieve When both Hd, ||5 are tuned so that the full amount of power is being utilized in the antenna, a complete transfer of energy occurs. Similarly, when one antenna is eii'ectively open circuited and the other is fully loaded, the full energy transmission from source |00 is taking place.

When the condition occurs sol that neither antennae I l2 or I3 are drawing any power from the load, an effective short circuit across network |02 at c and d occurs. Since |0| .is equal to an odd number of quarter wavelengths, and also arms itil, are equal to an odd number oi quarter wavelengths, this makes the total eiect oi transmission line 0| and arms |04, |05 equivalent to a short circuited half-wave transmission line. This is equivalent to a short circuit across terminal 2| 0 of the tuned secondary circuit 20| Since 20| is tuned to the operating frequency lwhen the effective short circuit is This open from the output 200 with respect were placed of the rotatable condenser.

vided with the proper load circuit arrangement.

Otherwise, if the system were permitted to attain such a position that a low output impedance l was provided at any time, the oscillator would carry such highy current as to probably result in a burning out of the oscillator.

For the purpose of providing the system trans..

mission arrangement such as described in connection with Fig. 2, the modulation arrangement must be of a type which will produce an effective opening of the circuit of lines ||0, so that the total impedance of either load varies between zero and For this purpose several types of arrangements may be utilized. In Figs. 3 and 4, I have illustrated two types of arrangements suitable for use in this arrangement. Referring rst to Fig, 3, two transmission line conductors 302 are shown. These may be the conductors of a line such as H0, of Figs. l and 2. Mounted close to these transmission line conductors is provided a transmission line section .303, closed at .one end and open circuited at the other. When such a transmission line section is coupled to a transmission line and tuned to the frequency of the system, the network will operate as a substantial cut-olf network, that is, substantially no energy will pass the ends of the network. However, when the network is even very slightly detuned from the operating frequency, it will have substantially no e'ect upon the transmission of currents to the line. Other forms of coupled networks may be used, the principles of such networks being more fully set forth in my United States Patent 2,159,648 of May 23, 1939. To perform the desired modulation arrangement a network such as shown at 30'! is mounted close to 30|, 302 so as to be coupled thereto, and between the open ends of this section I provide a rotatable condenser plate 330. provided with a plurality of notches, so that as the plate is rotated section 301 will be periodically tuned and detuned from the operating frequency. The frequency of the modulation signals may be determined by the speed of rotation of the condenser disc 330, and the number of notches provided therein. Thus, any desired modulation frequency may be produced. For mounting this arrangement it is convenient to provide a trough section such as 308 mounted around the transmission line conductors and carrying the shaft In order to prevent disturbances of the system by means of dirt accumulation, and so forth, a cover plate may be provided over the trough 308.

In Fig. 4 is shown another type of arrangement for producing the desired modulation effect. A section of transmission line 4| is bridged across the transmission lines 30|, 302, at a point preferably an integral multiple of a half wavelength from the junction of the bridge arm.

This condenser plate isV 'I'his network may be tuned to the operating frequency and detuned therefrom. When properly tuned it may operate so as to produce a virtual short circuit across the transmission line. Since, as stated above, this section may be spaced at a distance equivalent to a half wavelength from ^the junction point of the bridge arm andmay be made equal to a half wavelength long, it is lclear that at this time the effect will be the same as .though the transmissionline were short circuited at a point a half wavelength distant from the junction. This will be effectively equivalent to a short circuit at the junction point, for example, at a point such as b in the arrangement f shown in Fig. 2.

- Although I have illustrated only two forms of modulation circuit for use in the arrangement in accordance with my invention, it is clear that any suitable tuning networks may be provided, it being necessary that the system be arranged so that the load varies from zero up to its maximum amount of For this purpose other known types of networks, such as combinations of inductance and capacities may be utilized in place of the picture arrangements I have shown. However, the illustrated arrangements are generally more easily constructed and are generally more accurate to use for ultra-high frequencies and for this reason are preferable in the higher frequency ranges.

Although I have illustrated my invention in connection with certain preferred embodiments thereof, it should be distinctly understood that these illustrations are merely by way of example. The cross-modulation prevention arrangement may be utilized with any desired type of high frequency energy load.

What I consider to be my invention and upon what I desire to secure protection is embodied in the accompanying claims.

What I claim is:

1. A high frequency system comprising a reentrant loop circuit, a high frequency energy source having a given surge impedance coupled to a point on said reentrant loop circuit, a network having substantially the same impedance as said surge impedance coupled to said loop circuit at another point, said coupling points dividing said loop into two parts, one of said parts being electrically a half wavelength or an odd multiple of a half wavelength longer than the other of said parts, high frequency loads of substantially equal impedance values coupled to said l loop substantially midway between the points of connection of said source and said network, and means for differently modulating the energy delivered from said source to respective ones of said loads.

2. A high frequency system according to claim 1, wherein said arms of said reentrant loop formed by connection of said source, said loads said ioop circuit at a xed point and a high frequency impedance network having an impedance substantially equal to the impedance of said source connected at another point of said loop, said connecting points dividing said loop into two parts, one of said parts being electrical/ly substantially a half wavelength or an odd multiple thereof longer than the other of said parts, and means for connecting each said load to said loop at points substantially equi-distant electrically between the points of connection of said source and said impedance network.

4. A high frequency transmission system comprising a source of high frequency energy, two high frequency loads of substantially equal impedance, a network for interconnecting said source and said loads and providing a conjugate relationship between said loads comprising, a reentrant bridge circuit having four arms of substantially equal electrical length, one of said arms having transposition therein, means for coupling said source to the junction point of two of said arms. a high frequency impedance means. means for coupling said impedance means to the Junction point of said vbridge circuit diagonally related to said rst named junction, means for coupling said loads respectively to the other junctions of the arms of said bridge, f

and means for modulating differentlylthe energy delivered to said loads.

5. A high frequency transmission system according to claim 4, wherein the arms of said reentrant bridge circuit are each equal to a quarter wavelength of the operating frequency or an odd multiple thereof, said means lfor coupling said loads to said bridge each comprising transmission line sections, and said modulating means each comprising arrangements for varying the impedance of the transmission line from normal value to a very high impedance value, said modulating means being located at a point in said transmission line a quarter of a wavelength or an odd multiple of a quarter wavelength from the connection point of said load coupling means and said bridge.

f 'ANDREW ALFORD. 

